1. Field of the Invention
The invention relates to a cryogenic freezer for freezing food products using liquid air. More particularly, the invention relates to a freezing apparatus and a method of freezing food products which uses a combination of liquid nitrogen and liquid oxygen as a cooling medium and avoids the need for continuously exhausting vapors from the freezer.
2. Description of the Related Art
Food chilling and freezing on a commercial scale has been achieved by freezer designs employing mainly either liquid carbon dioxide, carbon dioxide "snow," or liquid nitrogen. However, these carbon dioxide or liquid nitrogen systems have inherent disadvantages in that nitrogen or carbon dioxide gases must be exhausted out of the freezer. The exhausting of cold exhaust gases from the freezer reduces the efficiency of the freezer. In addition, in both carbon dioxide and liquid nitrogen systems the leakage of exhaust gases into the room air may create unsafe conditions for workers.
Individually quick frozen ("IQF") food products refer to food products which are frozen individually prior to packaging, as opposed to products which are frozen in clumps or blocks. Examples of such IQF products include hamburger patties, fruits, vegetables, diced ham, chicken pieces and patties, shrimp, and scallops. Products frozen by IQF freezing techniques offer advantages over block freezing methods because these products can be used in the amount desired and the unused portion can be returned to the freezer for later use.
Different types of apparatus are known to be used for freezing of IQF food products including straight tunnel freezers, liquid nitrogen immersion freezers, carbon dioxide flighted freezers, carbon dioxide tumbling freezers, and mechanical chilled fluidized bed freezers. Each of these types of freezers, with the exception of mechanical freezers, uses either liquid carbon dioxide or liquid nitrogen as a cooling medium.
A traditional means of cryogenic freezing is through the use of what is referred to as a straight tunnel freezer. This process uses a conveyor belt that runs through an enclosed freezer. The product travels on the belt through the freezer. As the product travels, it is sprayed directly with cryogen and is cryogenically frozen. Typical cryogens used are liquid carbon dioxide (-110.degree. F., -79.degree. C.) and liquid nitrogen (-320.degree. F., -196.degree. C.). The large difference in temperature between the product and the gaseous atmosphere in the freezer causes a quick heat exchange and results in fast freezing of the product. Because of this quick freeze, or cryogenic freeze, it is known that there is less cell damage in the product and thus less product damage. In the straight tunnel freezer, there are usually several top mounted fans blowing down on the product causing the thin layer of warm air surrounding the product to be removed and replaced with the colder cryogen gases. This results in a quick transfer of heat from the product. This process works well with products that are large, mostly dry, and can be placed on the belt separated from each other so that they cannot freeze together.
The utilization of liquid carbon dioxide or liquid nitrogen in a straight tunnel freezer requires that the gaseous vapors be exhausted from the freezer. With liquid nitrogen freezers it is important to monitor the oxygen content of the area surrounding the freezer where workers are present to assure that the oxygen content of the air does not drop below a safe level of 18%. With liquid carbon dioxide freezers the concentrations of carbon dioxide in the room must be monitored to make sure they do not exceed tolerable exposure concentrations. An additional drawback of straight tunnel freezers is that the cooling capacity of the liquid carbon dioxide or liquid nitrogen is not fully used when the cold gaseous nitrogen or carbon dioxide vapors are exhausted from the freezer, thus reducing the efficiency of the freezer.
Tunnel freezers of other shapes, such as spiral or multi-layer, are also known which have some space savings and increased efficiency over straight tunnel freezers due to their more compact shapes. These other types of tunnel freezers have the same disadvantages as the straight tunnel freezers due to the use of liquid carbon dioxide or liquid nitrogen as a cooling medium.
The liquid nitrogen immersion freezer manufactured by Koach Engineering is a relatively compact device which provides freezing quickly and in a short distance by direct immersion of food product in liquid nitrogen. With this freezer, the product travels on a belt in a downward inclined direction into a bath of liquid nitrogen at -320.degree. F. (-196.degree. C.) where the partial fluidization and crusting of the product immediately take place. The product then travels in an upwardly inclined direction out of the freezer. This is the fastest form of individual quick freezing available. If the product particles are not touching each other when freezing, the product will be individually quick frozen. In addition, products can be of varying sizes and shapes and little floor space is required for this freezer because of the low operating temperature.
Efficiency of a liquid nitrogen immersion freezer can be improved by adding a postcooling tunnel to extract BTU's from the unused nitrogen vapors. A postcooling tunnel is a straight tunnel that pulls in the cold vapors being exhausted by the immersion freezer and recirculates them around the product, further cooling the frozen product.
The liquid nitrogen immersion freezer is subject to the same reduced efficiency and safety concerns associated with the use of liquid nitrogen which were discussed with respect to the tunnel freezers. In addition, this freezing method does not use the chilling potential of the gas vapors and thus more significantly decreases the efficiency. Even if a postcooling tunnel is used to improve the efficiency of the system, a substantial amount cooling ability of the cold nitrogen vapor is lost due to the need to exhaust these vapors.
The carbon dioxide flighted freezer made mainly by Liquid Carbonic Corporation provides a series of conveyor belts covered with a thin bed of carbon dioxide "snow." At the entrance end of the flighted freezer, there is a crust freezing zone where the product is sprayed with liquid carbon dioxide. The sprayed carbon dioxide creates a bed of dry ice snow in which the product travels along through the freezer. This bed of dry ice sublimes to help further freeze the product. The freezer tunnel has a series of belts which run at upward inclines. These inclined belt segments successively drop the food onto the next lower belt segment, dropping and tumbling the product and carbon dioxide snow together through the freezer. The purpose of these successive drops is to break apart product that is frozen together. Unfortunately, this is not always successful and the impact created from the drop does not always separate the product that has frozen together. The fans above the belt attempt to remove heat from the product and sublime the carbon dioxide snow before it exits the freezer.
Additional drawbacks of the carbon dioxide flighted freezer include the fact that carbon dioxide is the only expendable refrigerant that can be used with this process. Excess solid carbon dioxide snow on the belt that does not sublime before it exits the freezer may be packaged with the product. This packaged carbon dioxide can expand in the package causing the package to burst. Product breakage is also a common occurrence in this freezer due to the tumbling action of the product. Inefficiency and inconsistent product quality are additional drawbacks of the carbon dioxide flighted freezer. In addition, the amount of floor space and equipment required to produce large production in a flighted freezer can be a disadvantage when space constraints and equipment costs at a production facility are a consideration.
The carbon dioxide tumbler made by AirCo. includes a long rotating cylindrical drum set on an angle and fed by a conveyor belt which loads product directly into the rotating drum. Baffles inside the rotating drum lift and spill the product and direct the product towards the exit end of the freezer. Liquid carbon dioxide is also injected into the tumbler near the entrance and provides a tumbling bed of carbon dioxide snow for the product to travel in. The disadvantages of this method are similar to the disadvantages of the carbon dioxide flighted freezer in that the product tends to break as it tumbles through the rotating drum freezer and that excess carbon dioxide snow is packaged with the product resulting in package breakage. Additionally, due to very little shearing action within the process, product may stick and accumulate on the inner surfaces of the drum. Furthermore, the cool carbon dioxide gas vapors are not used efficiently due to the lack of ventilated gas movement.
All of the freezers which have been described above have the drawbacks associated with the use of liquid nitrogen or liquid carbon dioxide as a cooling agent. These freezers all require the continuous exhausting of the gaseous vapors which causes loss of efficiency and safety concerns due to possible leaks in the freezers.
The mechanical chilled fluidized bed freezers which are manufactured mainly by Frigoscandia, York and Ross, employ a freon or ammonia cooling system for cooling the air within the freezer through which the food products pass on a conveyor. The food product is levitated by using fans or blowers which force mechanically refrigerated air upward through the conveyor belt. This creates a "fluidized" bed of cold air around the product which promotes heat transfer and thus individually quick freezes the product. Advantages of mechanical freezing are the low operating cost, high production rates, and efficiency due to recirculation of cold gases.
The main drawbacks of the mechanical fluidized bed freezer include the fact that product needs to be lightweight, of uniform shape, and small size for the true fluidization to occur. In addition, the complexity of the system usually requires special attention for installation and maintenance. Furthermore, frequent downtime is required to defrost the ice build-up on condensation coils inside the mechanical freezer. Other drawbacks include high capital investment, inflexibility of increased or decreased production requirements and clumping if the product is not properly fed into the freezer.
In addition to this literature largely dedicated to processes and apparatuses implementing liquid nitrogen or liquid carbon dioxide, it has been proposed to use liquid air as a cryogenic liquid refrigerant in cryogenic food freezers (see for example U.S. Pat. No. 4,475,351 and U.S. Pat. No. 4,726,195).
One of the problems raised by handling such liquid air atmospheres comes from the fact that nitrogen is more volatile than oxygen, leaving a liquid enriched in oxygen, causing both risks of unsafe environmental conditions for workers around, and risks of detonation when this oxygen-enriched atmosphere comes in contact with a combustible or ignition source.
U.S. Pat. No. 4,726,195, cited above, offers a solution to those technical problems, wherein the liquid air is introduced inside the refrigeration zone of the freezer through a specific injector allowing that essentially all of the liquid cryogen is fully vaporized before contacting the product. Ensuring a full vaporization of both the nitrogen and the more slowly evaporating oxygen, before the liquid comes into contact with the products, eliminates the risks of detonation and unsafe working conditions for workers within the room in which the freezer is located.
This document has therefore made the choice of a freezing process using a cold gas.